For conservatives, the issue is pretty simple. The institution of marriage predates any existing government or nation. So no government has a right to redefine marriage. But it is okay for the government to become entangled with marriage towards the end of promoting marriage. The institution is good. It tames men. It protects women. It is good for children. Therefore, it is worth promoting.

Interesting thought; I will have to remember that. Since Government didn't create marriage, it's not theirs to redefine.